Gas sensitive materials for gas detection and method of making

ABSTRACT

A gas sensitive material comprising SnO2 nanocrystals doped with In2O3 and an oxide of a platinum group metal, and a method of making the same. The platinum group metal is preferably Pd, but also may include Pt, Ru, Ir, and combinations thereof. The SnO2 nanocrystals have a specific surface of 7 or greater, preferably about 20 m2/g, and a mean particle size of between about 10 nm and about 100 nm, preferably about 40 nm. A gas detection device made from the gas sensitive material deposited on a substrate, the gas sensitive material configured as a part of a current measuring circuit in communication with a heat source.

The invention was made with Government support under ContractDE-AC0676RLO 1830, awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

TECHNICAL FIELD

This invention relates to methods and apparatus for detecting gasses.More specifically, this invention relates to improved materials for usein gas sensing devices and methods of making the same.

BACKGROUND OF THE INVENTION

There have been numerous examples of instruments and methods fordetecting and measuring specific gases present in an atmosphere. Forexample, microcalorimetric gas sensors, (pellistors) burn combustiblegases with the surrounding air on the surface of a small ball or film ofcatalytically active metal. The catalyst, e.g. Pt, Pd, or Rh is kept at500-600° C. The heat of combustion in the presence of a gas is balancedby a reduction in electrical heating power. The power consumption servesas the signal indicating a concentration of flammable gases. This typeof sensor is the current standard for the detection of explosives inplants, because it shows a higher accuracy and longer-term stabilitythan the (cheaper) oxidic extensor prevailing in-home applications forthe same purpose. Examples include those shown in Debeda, H, Rebiere D,Pistre J, and Menil J 1995 Sensors Actuators B 27 297-300.

Electrochemical gas cells ionize the gas molecule at a three phaseboundary layer (atmosphere, electrode of a catalytically activematerial, electrolyte). Some examples of electrode materials areplatinum for CO, gold for a NO₂, and activated coal for SO₂ detection.Examples of these cells are shown in Brailsford A D, Yussougg M andLogothetis E M 1992 Technical Digest of the 4^(th) Meeting of ChemicalSensors (Tokyo) ed N Yamazoe (Japan Association of Chemical Sensors) p160.

Mass sensitive piezoelectric sensors detect a weight change of anabsorbtive layer by use of a quartz microbalance or a surface acousticwave substrate. Examples of these devices are described in Lucklum R,Hauptmann P 2000 Sensors Actuators B 70 30-6.

Field effect transistors (FET) have also been used as gas sensingdevices. Typically, in these arrangements, the gate metal is exposed tothe surrounding atmosphere and hydrogen or hydrogen containing gasesdisassociate or decompose on the surface and the protons defuse to themetal/insulator interface and influence the charge in the semiconductor,thereby changing the drain source current. Examples of such arrangementsinclude those described in Tobias P, Martensson P, Baranzahi A,Solomonsson P, and Lundstrom I 1998 Sensors Actuators B 47 125-30 andLampe U, Gerblinger J and Meixner H, 1992 Sensors Actuators B 7 787-94.

A crucial aspect of the preparation of gas sensors is the deposition ofthe sensing layer on a substrate surface. Known methods for thedeposition of this sensing layer include paste/slurry deposition,chemical vapor deposition (CVD), and physical vapor deposition (PVD).The various chemical and physical vapor deposition (CVD or PVD)techniques are mostly standard processes in the semiconductor industry,the liquid deposition techniques are less frequently employed. However,the compatibility of the latter, i.e. screenprinting and drop depositiontechniques with semiconductor processes have been shown to be feasible.

One example of a gas sensor is shown in U.S. Pat. No. 5,470,756 issuedto Coles et al. Nov. 28, 1995. All documents, patents, journal articles,and other materials cited in the present application are herebyincorporated by reference.

As described by Coles, a gas sensitive layer is formed of SnO₂incorporating BiO₃ in an amount less than 35%, but sufficient to conferhydrogen sensitivity and selectivity. Coles further contemplates theinclusion of the catalyst selected from the group of metals Ir, Pt, Ag,Ru, Au or Pd. Coles teaches the deposition of these materials on asubstrate as a slurry.

Drawbacks of the prior art methods include slow response times, lowsensitivity, high manufacturing costs, and difficulty in reproducingconsistent results. Accordingly, new materials and methods offabrication are needed to improve gas sensors. The present invention isdirected towards such materials and methods.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a gassensitive material which exhibits a rapid change in conductivity in thepresence of reducing gases, including, but not limited to, H₂, CO, CH₄,NH₃ and combinations thereof. Another object of the present invention isto provide a gas sensitive material comprising SnO₂ nanocrystals dopedwith In₂O₃, and an oxide of a platinum group metal. While it ispreferred that Pd, or a combination of Pd with any of Pt, Ru, Ir, beselected as the platinum group metal, suitable platinum group metalsinclude Pd, Pt, Ru, Ir, and combinations thereof.

A further object of the present invention is to provide the gassensitive material wherein the SnO₂ nanocrystals have a specific surfaceof 7 m²/g or greater. Yet another object of the present invention is toprovide a gas sensitive material wherein said SnO₂ nanocrystals have aspecific surface of about 20 m²/g. In a preferred embodiment of thepresent invention, the gas sensitive material of SnO₂ nanocrystals havea mean particle size of between about 10 nm and about 100 nm. In anotherpreferred embodiment of the present invention, the gas sensitivematerial of SnO₂ nanocrystals have a mean particle size of about 40 nm.

These and other objects of the present invention are met by providing amethod of forming a gas sensitive material wherein a mixture of SnO₂,In₂O₃, and an oxide of a platinum group metal is heated to a temperaturesufficient to form nanocrystals.

As used herein, it should be understood and recognized that in theprocess of forming the mixture that ultimately forms the gas sensitivematerial, these precursor materials are not necessarily provided intheir final form. For example, it is typically convenient to provide thepreferred platinum group metal, as a salt. For example, in the case ofPd, Pd is provided as a salt. The salt is then placed in solution, whichis then treated to disassociate the Pd atoms. The Pd is oxidized by thesurrounding water vapor to form PdO.

Those having ordinary skill in the art will recognize that on occasionincomplete oxidation of the platinum group metal will occur.Accordingly, some fraction of the platinum group metal may be present inthe final gas sensitive material in an unoxidized form. Further,operation of a device incorporating the gas sensitive material may causethe reduction of the metal oxide. Thus, it should be understood that thepresence of some fraction of the platinum group metal in an unoxidizedform in the final gas sensitive material is expressly contemplatedherein.

Preferably, but not meant to be limiting, the oxide of the platinumgroup metal comprises between about 2% and about 5% of the weight of theSnO₂ nanocrystals and the In₂O₃ comprises between about 3% and about 12%of the weight of the SnO₂ nanocrystals. More preferred, and still notmeant to be limiting, the oxide of the platinum group metal comprisesabout 3% of the weight of the SnO₂ nanocrystals and the In₂O₃ comprisesabout 6% of the weight of the SnO₂ nanocrystals.

In a preferred embodiment of the present invention, an additive isprovided in the mixture of SnO₂, oxide of the platinum group metal, andIn₂O₃. It is preferred, but not meant to be limiting, that the additivecomprises a surfactant, a blowing agent, and combinations thereof. Inthis embodiment, the surfactant comprises between about 8% to about 20%of the mixture by weight and the blowing agent comprises between about3% and about 6% of the mixture by weight. Even more preferred, but notmeant to be limiting, the surfactant comprises about 15% of the mixtureby weight and the blowing agent comprises about 5% of the mixture byweight.

While not meant to be limiting, ammonium carbonate is preferred as ablowing agent. Upon heating, ammonium carbonate decomposes to a gasform, and is thereby removed from the mixture as CO₂ and NH₃. Othersuitable compounds for use as a blowing agent include, but are notlimited to, the azo-compounds (which decompose with liberation of N₂),and ammonium chloride (which decomposes with formation of NH₃ and HCl).

Also, while not meant to be limiting, it is preferred that thesurfactant be stearic acid. As with the blowing agent, the surfactant isalso decomposed to a gas form and thereby removed from the mixtureduring the formation of the gas sensitive material. Other suitablesurfactants include, but are not limited to, carbonic acids with longcarbonic chains, and non-ionic surfactants such as monolaureate (Tween20, Tween 21, Span 20), monopalmitate (Tween 40, Span 40), monostearate(Tween 60, Tween 61, Span 60), tristearate (Tween 65, Span 65),monooleate (Tween 80, Tween 81, Span 80) and trioleate (Tween 85, Span85).

In a preferred embodiment, the present invention utilizes the gassensitive material of SnO₂ nanocrystals doped with PdO and In₂O₃ in agas detection or gas sensing device. (As used herein, the terms “gasdetection” and “gas sensing”, should be interpreted as beingsynonymous). As a part of a gas detection device, the gas sensitivematerial is deposited on a substrate, and is configured as a part of acircuit. By measuring the current, or changes in the current, throughthat circuit gases may be detected, and the relative quantities of thosegasses measured.

While not meant to be limiting, in one embodiment of the gas detectiondevice, the substrate of the gas detection device is in communicationwith a heat source. This embodiment may include, for example, aconfiguration where the heat source is a layer of material bonded to thesubstrate and is configured to be resistively heated as part of aheating circuit. In this manner, the gas sensitive material may bemaintained at an optimal or constant temperature while the currentflowing through the gas sensitive material is measured.

FIGS. 1 and 2 depict an illustrative arrangement of a gas detectiondevice, indicated generally as (100). Substrate (110) has a top (116)and bottom (118), a substrate first end (112) and a substrate second end(114). A gas sensitive material (130) is deposited on the substrate top(116). Electrical contact (142) is proximal the substrate first end(112) and the electrical contact (144) is proximal the substrate secondend (114). A heater layer (120) is deposited on the substrate bottomsurface (118).

Typically, but not meant to be limiting, the substrate (110) is adielectric plate, for example SiO₂ or Al₂O₃ (such as sapphire, orpolycor) with dimensions of about 0.5 mm thickness and a width andlength in the range of 10×10 mm to 1×1 mm. Also typical, but not meantto be limiting, the heater layer (120) is a Pt-layer deposited on thebottom surface (118) of the substrate (110). The gas sensitive material(130) may be deposited on the top surface (116) of the substrate (110)opposite the heater layer (120).

While general description of the present invention has herein beenprovided, a detailed description of experiments which have reduced theinvention to practice and demonstrated its advantages and benefitsfollows. These experiments, and the specific embodiments describedtherein, should in no way be viewed as limiting the scope of theinvention. Rather, the description of these experiments should berecognized as being merely demonstrative in nature. Those havingordinary skill in the art will readily appreciate that numerousalternatives to the specific details set forth in these experimentsexist, and that these alternatives will achieve the same results as theexperiments described herein without departing from the spirit and scopeof the invention. Accordingly, the scope of the present invention,should in no way be viewed as limited by the detailed description whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the inventionwill be more readily understood when taken in conjunction with thefollowing drawing, wherein:

FIG. 1 is an illustration of one possible arrangement of the gassensitive material within a gas detection device of the presentinvention.

FIG. 2 is an illustration of one possible arrangement of the gassensitive material within a gas detection device of the presentinvention.

FIG. 3 (a) is an AFM picture showing the particles of the gas sensitivematerial of the present invention. FIG. 3 (b) is X-ray diffraction dataindicating that the size of nanocrystals of the gas sensitive materialof the present invention is the range 15-40 nm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An experiment was conducted to fabricate one exemplary embodiment of thegas sensitive layer of the present invention. SnO₂ nanopowder fromAldrich (ref. 54, 965-7) with particles of mean size of about 40 nm wasblended with In₂O₃ nanopowder and ground in a ball mill with corundumballs for a period of 2-3 hours. The prepared mixture was added to anaqueous solution containing palladium chloride. The Pd-content in themixture was about 3%. The mixture was then thermally treated attemperatures close to 100° C. Heating the aqueous solution of PdCl₂ atthis temperature results in the salt decomposing in the presence ofwater vapors and PdO is thereby formed. Although the rate of thisreaction increases with temperature, at the same time the increase oftemperature leads to a rapid loss of water from the composition, which,at some point, would prevent the conversion of PdCl₂ to PdO. Therefore,about 100° C. is preferred temperature for salt decomposition as athigher temperatures water can be vaporized before the completion of thedecomposition reaction.

The mixture was then dried at a temperature of about 100° C. for aperiod of about 0.5-1 hour. The dried mixture was then blended with a 5%solution of ethyl cellulose in terpineol containing a surfactant ofstearic acid (about 15% by weight) and blowing agent of ammoniumcarbonate (about 5% by weight). The powder blend was carefully stirredfor a period of 2-3 hours. The surfactant and a blowing agent wereintroduced into the resultant paste in order to modify the morphology ofthe gas sensitive layer to increase the layer porosity, that is, thepore volume and specific surface area of the layer. A thin sheet of thepaste-like mixture was laid on a substrate and annealed at a temperatureof about 550° C.

The sample was slowly heated up to 550° C. (with a heating rate of about2° C./min) and maintained at this temperature for about 2.5-3 hours toachieve the stationary value of the gas sensitive layer conductivity.The thermal treatment produces a sintered gas sensitive layer stronglyadhered to the substrate. During the thermal treatment, organic bindersof the paste (solution ethyl cellulose in terpineol), as well as stearicacid and ammonium carbonate are fully converted to gaseous products andthereby removed from the gas sensitive layer.

PdO-clusters are very quickly reduced to metallic Pd-clusters in thepresence of H₂ at temperatures between 400-450° C., which is a thepreferred temperature range for use of the gas sensitive layer in gasdetection devices. Dissociation of H₂ molecules to H-atoms proceedsalmost entirely on these Pd-clusters, which are the active catalyst inthis reaction. H-atoms, formed on Pd-clusters, transfer to SnO₂ andreact with 0″ adsorbed on surface oxygen vacancies of SnO₂. The reactionof H₂ with adsorbed O⁻ in Pd-doped SnO₂ can be presented by thefollowing scheme:PdO+H₂→Pd+H₂O  (1)Pd+H₂

Pd+2H(ad,Pd)  (2)H(ad,Pd)→H(ad,SnO₂)  (3)2H(ad,SnO₂)+O⁻(ad,SnO₂)→H₂O+e ⁻(vacancy,SnO₂)  (4)

The main factors influencing sensitivity are: the dissociation degree ofH₂ to H-atoms and the rate of the reaction H-atoms with adsorbed O⁻resulting in the liberation of conductive electrons. Doping SnO₂ with Pdincreases the equilibrium degree of H₂ dissociation to H-atoms anddecreases the time to achieve this equilibrium. These effects result inthe rise of the electron liberation rate and in the correspondingincrease of sensor sensitivity and sensor response rate of Pd-dopedSnO₂-sensors.

The gas sensitive layer produced in these experiments thus consisted ofnanocrystals SnO₂ with In₂O₃ and PdO dopants. Data on the half-widthvalues of X-ray diffraction peaks showed that the mean size of SnO₂- andIn₂O₃-nanocrystals is in the range 15-40 nm. The specific surface of thegas sensitive layer, as measured by the argon adsorption (BET-method),was about 10-14 m²/g. An atomic force microscope (AFM) was used tocharacterize the size of the particles on the sensor layer surface. Thepicture is shown as FIG. 3.

While a preferred embodiment of the present invention has been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

1. A method of forming a gas sensitive material comprising the steps of:providing a mixture of SnO₂, InO₃, and an oxide of a platinum groupmetal; heating said mixture to a temperature sufficient to formnanocrystals; and forming a multicomponent paste from the mixture, andwherein the heating comprises annealing the mixture.
 2. The method ofclaim 1 further comprising the step of providing said platinum groupmetal as Pd, Pt, Ru, Ir, and/or combinations thereof.
 3. The method ofclaim 2 further comprising the step of providing said platinum groupmetal as Pd.
 4. The method of claim 1 further comprising the step ofproviding an additive in said mixture of SnO₂, InO₃, and an oxide of aplatinum group metal.
 5. The method of claim 4 wherein said additivecomprises a surfactant, a blowing agent, and/or combinations thereof. 6.The method of claim 5 wherein the surfactant comprises between about 8%to about 20% of said mixture by weight and said blowing agent comprisesbetween about 3% and about 6% of said mixture by weight.
 7. The methodof claim 5 wherein the surfactant comprises about 15% of said mixture byweight and said blowing agent comprises about 5% of said mixture byweight.
 8. The method of claim 4 wherein the surfactant is stearic acidand the blowing agent is ammonium carbonate.
 9. The method of claim 1wherein said oxide of a platinum group metal comprises between about 2%and about 5% of the weight of said mixture and said InO₃ comprisesbetween about 3% and about 12% of the weight of said mixture.
 10. Themethod of claim 1 wherein said oxide of a platinum group metal comprisesabout 3% of the weight of said mixture and said InO₃ comprises about 6%of the weight of said mixture.
 11. The method of claim 1 wherein theproviding mixture of SnO₂, InO₃, and an oxide of a platinum group metal,comprises: blending a Pd salt solution with SnO₂ and/or In₂O₃ particlesto associate the Pd with the particles, and converting the Pd salt to adispersed Pd oxide; and adding a surfactant and processing to produce agas sensitive material.
 12. A method of forming a gas sensitive materialcomprising the steps of: providing a mixture of SnO₂, In₂O₃, and anoxide of a platinum group metal; heating said mixture to a temperaturesufficient to form nanocrystals; adding at least one of a surfactant anda blowing agent to form a multi-component paste composition; andannealing the paste composition to remove one or both of the surfactantand the blowing agent.
 13. The method of claim 12 wherein the surfactantcomprises between about 8% to about 20% of the mixture of SnO₂, InO₃,and an oxide of a platinum group metal by weight and the blowing agentcomprises between about 3% and about 6% of the mixture by weight. 14.The method of claim 12 wherein the multi-component paste composition isformed as a film.